As well known in the art, bonded magnets have many advantages such as light weight, good dimensional accuracy, and facilitated mass-production of molded products having even a complicated shape as compared to sintered magnets, and, therefore, have been extensively used in various applications such as toys, office equipments, audio equipments and motors.
As the magnetic particles usable in the bonded magnets, there are known rare earth element magnet particles such as typically Nd—Fe—B-based alloy particles, or ferrite particles. The rare earth element magnet particles have high magnetic properties, but are expensive, resulting in limited applications thereof. On the other hand, the ferrite particles are somewhat deteriorated in magnetic properties as compared to the rare earth element magnet particles, but are inexpensive and chemically stable and, therefore, have been used in more extensive applications.
The bonded magnets have been usually produced by kneading a rubber or a plastic material with magnetic particles and then molding the resulting kneaded material in a magnetic field or by using a mechanical means.
In recent years, with the enhancement in performance of various materials or equipments including an improved reliability, there is also an increasing demand for a high performance of bonded magnets used therein including enhancement in strength and magnetic properties of the bonded magnets.
More specifically, the bonded magnet molded products obtained by injection molding, etc., are also required to exhibit a good mechanical strength capable of withstanding severe conditions when used in various applications.
For example, in the applications of motors in which a rotating rotor having a shaft is used, since the shaft is inserted into the rotor machined into various sizes and complicated shapes, the rotor is required to have a high tensile elongation characteristic. With respect to the magnetic properties of the rotor, there have been required not only an increased level of a surface magnetic force but also a less degree of demagnetization induced owing to a magnetic field generated by a current supplied from a coil, in particular, as an important factor technology for obtaining a high-performance motor characteristics.
For this reason, ferrite particles used in the bonded magnets as well as resin compositions for the bonded magnets which comprise the ferrite particles and an organic binder are also required to satisfy the above requirements.
Conventionally, ferrite particles for bonded magnets and resin compositions for bonded magnets which comprise the ferrite particles and the organic binder have been improved variously. For example, there are known the method of producing ferrite particles by using an alkaline metal compound or an alkaline earth metal compound as a flux (Patent Literature 1); the method of controlling a particle size distribution of ferrite particles (Patent Literature 2); the method of producing a bonded magnet using ferrite magnetic particles comprising an alkaline earth metal as a constituting component and having an average particle diameter of not less than 1.50 μm and a melt flow rate of not less than 91 g/10 min (Patent Literature 3); the method of controlling properties of compacted calcined particles obtained by producing particles having an average particle diameter of not more than 2.5 μm and a specific surface area of not less than 1.25 m2/g and then subjecting the resulting particles to annealing and further to compaction, so as to satisfy the conditions of Ra<2.5 μm and Ra-Da<0.5 μm wherein Ra (μm) represents an average particle diameter of the particles as measured by a dry air dispersion laser diffraction method, and Da (μm) represents a specific surface area diameter of the particles as measured by an air permeability method (Patent Literature 4); the method of obtaining a ferrite having a large particle diameter, a clear crystal structure, a coercive force that is hardly reduced even when pressed, and an energy product of not less than 2.0 MGOe (Patent Literature 5); or the like.